Directional de-signature for seismic signals

ABSTRACT

The present invention provides a method and apparatus for directional de-signature of a seismic signal. The method includes forming a plurality of far-field signatures representative of a plurality of seismic signals having a plurality of take-off angles, associating a plurality of traces representative of a plurality of reflections of the seismic signals with the plurality of far-field signatures, and forming a plurality of de-signatured traces from the plurality of traces and the plurality of associated far-field signatures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to seismic surveys, and, moreparticularly, to directional de-signature for seismic signals.

2. Description of the Related Art

Underwater seismic exploration is widely used to locate and/or surveysubterranean geological formations for hydrocarbon deposits. A surveytypically involves deploying one or more seismic sources and one or moreseismic sensors at predetermined locations. For example, a seismic cableincluding an array of seismic sensors and a seismic source may each betowed along the ocean's surface by a survey vessel. A seismic signal, orshot, provided by the seismic sources generates an acoustic signal thattravels to the geological formations, where the acoustic signal isreflected and propagates back to the seismic sensors. The seismicsensors receive the reflected signals, which are then processed togenerate seismic data, or traces. Analysis of the traces may indicateprobable locations of geological formations and hydrocarbon deposits.

A representation of the acoustic signal known as a signature may also beformed. For example, a so-called far-field signature that isrepresentative of a portion of the acoustic signal that is received bythe seismic sensor may be calculated. Historically, an estimate of thefar-field signature is removed from the seismic data to reduceinterference, a process known as “de-signaturing.” For one example, amodel may be used to estimate the far-field signature and de-signaturethe seismic signal. For a second example, a statistical estimate of thefar-field signature may be calculated based upon previous data and thestatistical estimate is used to de-signature the seismic signal.

In traditional de-signaturing processes, the far-field signature isestimated by assuming that all the energy in the shot leaves the seismicsource and travels vertically downwards. A vertical de-signaturing maythen be performed using the estimated far-field signature. However, notall of the far-field signature data that is recorded with the seismicdata travels vertically from the seismic source to the receiver. Inreality, the energy in the shot may leave the seismic source along arange of takeoff angles and may arrive at the receivers along a varietyof emergent angles. Neglecting the takeoff and emergent angles of thesignatures may reduce the accuracy of the de-signaturing process. Inparticular, the phase and high-frequency power of the far-fieldsignature may be incorrectly calculated, which may, in turn, lead to areduction in the resolution of the seismic survey.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method is provided fordirectional de-signature of a seismic signal. The method includesforming a plurality of far-field signatures representative of aplurality of seismic signals having a plurality of take-off angles,associating a plurality of traces representative of a plurality ofreflections of the seismic signals with the plurality of far-fieldsignatures, and forming a plurality of de-signatured traces from theplurality of traces and the plurality of associated far-fieldsignatures.

In another aspect of the instant invention, an apparatus is provided fordirectional de-signature of a seismic signal. The apparatus includes astorage unit for storing data representative of a seismic signal, aplurality of traces representative of a plurality of reflected seismicsignals, and a plurality of take-off angles and at least one notionalsignature corresponding to the seismic signal. The apparatus alsoincludes a processor capable of forming a plurality of de-signaturedtraces using the seismic signal, the plurality of traces, the take-offangles, and the at least one notional signature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 conceptually illustrates a seismic survey system;

FIGS. 2A-B each show aspects of a rack-mounted computing apparatus thatmay be used in the seismic survey system shown in FIG. 1;

FIG. 3 conceptually illustrates various signals that may be provided inthe seismic survey system illustrated in FIG. 1;

FIGS. 4A and 4B conceptually illustrates angle-dependent signatures thatmay be formed using the various signals shown in FIG. 3;

FIG. 5A illustrates a directional de-signaturing process that may beused in the system shown in FIG. 1;

FIG. 5B illustrates a first exemplary process for applying directionalde-signature filters that may be used in the directional de-signaturingprocess shown in FIG. 5A;

FIG. 5C shows a second exemplary process for applying a two-dimensionaldirectional de-signature filer that may be used in the directionalde-signaturing process shown in FIG. 5A;

FIG. 6 conceptually illustrates interference signals that may beprovided in the seismic survey system illustrated in FIG. 1; and

FIG. 7 illustrates one embodiment of a process for removing a receiverghost signal.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

FIG. 1 conceptually illustrates a seismic survey system 100 comprising atowed receiver array 101 and a towed source array 102, in accordancewith one embodiment of the present invention. A seismic survey vessel105 tows a seismic streamer 110 by way of a first tow cable 115. It willbe appreciated that the seismic survey system 100 may be used in anydesirable environment. For example, in various alternative embodiments,the seismic survey system 100 may be used in bodies of water includingoceans, seas, fresh water, brackish water, and the like.

The streamer 110 may comprise a tail buoy 120. The tail buoy 120typically identifies the end of the streamer 110. The streamer 110 isadditionally provided with one or more levelling devices or “birds” 125that regulate the depth of the streamer 110 within the water. Thestreamer 110 also includes one or more seismic receivers 140. In oneembodiment, the seismic receivers 140 are hydrophones, but the presentinvention is not so limited. In alternative embodiments, the seismicreceivers 140 may be any desirable receiver. Furthermore, it will beappreciated by those of ordinary skill in the art that the number ofbirds 125 and receivers 140 is a matter of design choice and notmaterial to the present invention.

The seismic survey vessel 105, by way of a second tow cable 130, alsotows one or more seismic sources 135, which generate an acoustic wave(not shown) in the water that generally travels in a downward directiontowards the sea bed (also not shown). In one embodiment, the seismicsources 135 may be airguns. However, the present invention is not solimited. In alternative embodiments, the seismic sources 135 may be anydevice capable of generating the desired acoustic wave, such aspiezoelectric devices, hydraulic vibrators, seismic source arrays, andthe like. The towed source array 102 also includes one or more receivers150, which may be coupled to the second tow cable 130. In oneembodiment, the receivers 150 are hydrophones, but the present inventionis not so limited. In alternative embodiments, the receivers 150 may beany desirable receiver. Furthermore, it will be appreciated by those ofordinary skill in the art that the number of seismic sources 135 andreceivers 150 is a matter of design choice and not material to thepresent invention.

In operation, the seismic sources 135 impart an acoustic wave throughthe water and into the ocean floor. The acoustic wave reflects andrefracts from various structures (also not shown) within the sea bed andabove the seabed, and the reflected and/or refracted wave (also notshown) is detected by the receivers 140 in the streamer 110. As is wellknown in the art, upon receipt of the reflected and/or refracted wave,the receiver 140 typically generates analogue signals. In oneembodiment, the analogue signals may be converted to digital signals bydigital-to-digital converters (not shown) in the streamer 110.

For the sake of clarity, FIG. 1 illustrates two towed arrays 101, 102comprising two tow cables 115, 130 and one streamer 110 attached to thefirst tow cable 115. However, any number of arrays may contain anynumber of streamers, in accordance with conventional practice. The twotowed arrays 101, 102 may further comprise devices not shown in FIG. 1,in accordance with conventional practice, such as a towed buoy or apositioning device. Furthermore, it should be appreciated that theseismic sources 135 and the receiver 140 may be towed by the same cable.In other embodiments, the seismic sources 135 may be placed on a mobileor semi-mobile unit (not shown) positioned some distance away from theseismic survey vessel 105. It should also be appreciated that, in oneembodiment, the seismic streamer 110 may be an ocean-bottom cable(“OBC”). OBCs may be deployed on the seafloor to record and relay datato the seismic survey vessel 105. OBCs generally enable surveying inareas where towed streamers 110 are unusable or disadvantageous, such asin areas of obstructions and shallow water inaccessible to ships. In analternative embodiment, the receivers 140 may be buried in the earth orplaced in a borehole.

A signal processing unit 132 is provided to process the analogue and/ordigital signals that are generated by the receivers 140. Although notrequired for the operation of the present invention, it will beappreciated by those of ordinary skill in the art that the signalprocessing unit 132 may comprise a data collection unit (not shown) forreceiving the analogue and/or digital signals that are generated by thereceivers 140 and a data processing unit (also not shown). In oneembodiment, the signal processing unit 132 is deployed on the seismicsurvey vessel 105. However, it will be appreciated by those of ordinaryskill in the art that portions of the signal processing unit 132 may belocated in any desirable location, including, but not limited to, othervessels (not shown) and on-shore facilities (not shown). For example, inone embodiment, the data collection unit may be deployed on the seismicsurvey vessel 105 and the data processing unit may be deployed at aremote on-shore facility.

The analogue and/or digital signals generated by the receivers 140 aretransmitted over the streamer 110 and the tow cable 130 to the signalprocessing unit 132. In various alternative embodiments, the analogueand/or digital signals are transmitted to the signal processing unit 132via electrical or optical wires, cables, or fibres. Thus, these analogueand/or digital signals may be, for example, electrical and/or opticalsignals. In another set of alternative embodiments, the analogue and/ordigital signals are transmitted to the signal processing unit viawireless transmission devices such as a radio-frequency transmitter andthe like. Furthermore, in yet another set of possible embodiments, theanalogue and/or digital signals can be stored and transmitted to thesignal processing unit 132 using any storage medium, including, but notlimited to, recording tape, magnetic disks, compact disks, and DVDs. Thesignal processing unit 132 uses the analogue and/or digital signals toform one or more traces representative of the analogue and/or digitalsignals, in a manner well known to those of ordinary skill in the art.

In an alternative embodiment not shown, the signal processing unit 132,or at least the data processing unit, is located at an on-shore facility(not shown). Accordingly, the signals generated by the seismic receivers140 and near-field receivers 150 may be stored on, e.g., the surveyvessel 105 for later processing. Some embodiments may also, in additionto or in lieu of storing the signals, transmit them to the on-shorefacility. This may be done, for example, over a satellite link. Thus, itis not necessary to the practice of the invention that these signals beprocessed at the point or site of their collection.

To collect information used to de-signature the traces, and therebyreduce or eliminate the contribution of the acoustic signal to thetraces, one or more near-field receivers 150 are positioned near theseismic sources 135, in accordance with one embodiment of the presentinvention. For example, the near-field receivers 150 may be hydrophonescapable of recording calibrated marine source data. As described in moredetail below, the data recorded by the near-field receiver 150 is usedto estimate a so-called “notional” signature, which may be used tocompute a plurality of angle-dependent far-field signatures. The signalprocessing unit 132 may use the plurality of angle-dependent far-fieldsignatures and the plurality of traces to form a plurality ofde-signatured traces. By using the angle-dependent far-field signaturesto de-signature the traces, the accuracy of the de-signaturing processmay be improved and the resolution of the seismic survey may beincreased.

Referring now to FIGS. 2A-B, a rack-mounted computing apparatus 200 thatmay be deployed on the survey vessel 105 to implement the signalprocessing unit 132 is shown. The computing apparatus 200 includes aprocessor 205 communicating with some storage 210 over a bus system 215.The storage 210 may include a hard disk and/or random access memory(“RAM”) and/or removable storage such as a floppy magnetic disk 217 andan optical disk 220. The storage 210 is encoded with a data structure225 storing the signals collected as discussed above, an operatingsystem 230, user interface software 235, and an application 265. Theuser interface software 235, in conjunction with a display 240,implements a user interface 245. The user interface 245 may includeperipheral I/O devices such as a key pad or keyboard 250, a mouse 255,or a joystick 260. The processor 205 runs under the control of theoperating system 230, which may be practically any operating systemknown to the art. The application 265 is invoked by the operating system230 upon power up, reset, or both, depending on the implementation ofthe operating system 230.

The rack-mounted computing apparatus 200 may be used to implement atleast a portion of the signal processing unit 132 (shown in FIG. 1).Consequently, some portions of the detailed descriptions herein arepresented in terms of a software implemented process involving symbolicrepresentations of operations on data bits within a memory in acomputing system or a computing device. These descriptions andrepresentations are the means used by those in the art to mosteffectively convey the substance of their work to others skilled in theart. The process and operation require physical manipulations ofphysical quantities. Usually, though not necessarily, these quantitiestake the form of electrical, magnetic, or optical signals capable ofbeing stored, transferred, combined, compared, and otherwisemanipulated. It has proven convenient at times, principally for reasonsof common usage, to refer to these signals as bits, values, elements,symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated or otherwise as may be apparent, throughout thepresent disclosure, these descriptions refer to the action and processesof an electronic device, that manipulates and transforms datarepresented as physical (electronic, magnetic, or optical) quantitieswithin some electronic device's storage into other data similarlyrepresented as physical quantities within the storage, or intransmission or display devices. Exemplary of the terms denoting such adescription are, without limitation, the terms “processing,”“computing,” “calculating,” “determining,” “displaying,” and the like.

Note also that the software implemented aspects of the invention aretypically encoded on some form of program storage medium or implementedover some type of transmission medium. The program storage medium may bemagnetic (e.g., a floppy disk or a hard drive) or optical (e.g., acompact disk read only memory, or “CD ROM”), and may be read only orrandom access. Similarly, the transmission medium may be twisted wirepairs, coaxial cable, optical fibre, or some other suitable transmissionmedium known to the art. The invention is not limited by these aspectsof any given implementation.

Referring now to FIG. 3, a schematic diagram illustrating varioussignals encountered in the operation of the seismic survey system 100 isshown. Two seismic sources 300(1-2) provide acoustic signals 305(1-2),respectively, that may be received by a receiver 310 after beingreflected by a floor 315. As discussed above, the received acousticsignals 305(1-2) are transmitted to the signal processing unit 132,which may form a plurality of traces from the received acoustic signals305(1-2). In the interest of clarity, the acoustic signals 305(1-2) aredepicted as reflecting from the floor 315. However, those of ordinaryskill in the art will appreciate that the acoustic signals 305(1-2) mayalso penetrate the floor 315 and may be reflected by the various strataand other geologic features beneath the floor 315. Furthermore, to avoidobscuring the relevant details, the tow cables 115, 130, the streamer110, and other components of the seismic survey system 100 are not shownin FIG. 3.

The seismic sources 300(1-2) also provide two direct arrival acousticsignals 320(1-2) to two respective near-field receivers 325(1-2). Thedirect arrival acoustic signal 320(1-2) may be received by therespective near-field receiver 325(1-2) and may form a portion of anear-field signature. In one embodiment, the near-field signature istransmitted to the signal processing unit 132, which uses the near-fieldsignature to estimate at least one so-called notional signaturerepresentative of the acoustic signals 305(1-2) near the seismic sources300(1-2). For example, the seismic source 300(1) provides the directarrival acoustic signal 320(1) to the near-field receiver 325(1).

The various signals 305(1-2) each have a corresponding take-off angle327, which is measured at the seismic sources 300(1-2). In oneembodiment, the take-off angles 327 are measured from the vertical, asindicated by the dashed lines in FIG. 3. The various signals 305(1-2)also each have a corresponding emergent angle 328, which is measured atthe receiver 310. In one embodiment, the emergent angles 328 aremeasured from the vertical, as indicated by the dashed lines in FIG. 3.If the floor 315 is exactly horizontal, the take-off angles 327 of theacoustic signals 305(1-2) are equal to the emergent angles 328. However,for dipping data, wherein the floor 315 is not generally horizontal, thetake-off angles 327 of the acoustic signals 305(1-2) may not be equal tothe emergent angles 328.

The signal processing unit 132 may use the at least one notionalsignature to form a plurality of angle-dependent far-field signatures400(1-4), as shown in FIG. 4A. However, it will be appreciated that theangle-dependent far-field signatures 400(1-4) depicted in FIG. 4A arestylised representations of actual angle-dependent far-field signatures400(1-4), which may include a variety of features not shown herein.Furthermore, it will be appreciated that the number of angle-dependentfar-field signatures 400(1-4) is a matter of design choice, driven inpart by the desired angular resolution, and is not material to thepresent invention.

Each angle-dependent far-field signature 400(1-4) corresponds to thetake-off angle 327 of the corresponding acoustic signal. In oneembodiment, the signal processing unit 132 may use a model to computethe plurality of angle-dependent far-field signatures 400(1-4) from theat least one notional signature. In an alternative embodiment, thesignal processing unit 132 may use statistical measures derived fromprevious shots to compute the plurality of far-field signatures 400(1-4)from the at least one notional signature. However, it will beappreciated that the present invention is not limited to a particularprocess for computing the far-field signatures 400(1-4), and anytechnique known in the art for estimating and/or computing the far-fieldsignatures 400(1-4) may be used.

Moreover, it will be appreciated by those of ordinary skill in the artthat the far-field signatures 400(1-4) may also vary in azimuth. Forexample, in one embodiment, the sources 300(1-2) and the receiver 310may line up along a north-south line. In another embodiment, such as maybe used in undershooting applications, the sources 300(1-2) and thereceiver 310 may line up along an east-west line. The response of boththe sources 300(1-2) and the receiver 310 will usually be different atdifferent azimuths. For example, the response of the sources 300(1-2)and/or the receiver 310 aligned in a north-south orientation may differfrom the response of the sources 300(1-2) and/or the receiver 310aligned in an east-west orientation. Thus, the angle-dependent far-fieldsignatures 400(1-4) may also, in one embodiment, reflect an azimuthaldependence.

As shown in FIG. 4B, the power in the angle-dependent far-fieldsignatures 400(1-4) may vary with take-off angle 327 and/or frequency.In one embodiment, the power in the angle-dependent far-field signatures400(1-4) decreases with increasing take-off angle 327. The decrease inpower may be present at substantially all frequencies. However, in oneembodiment, the decrease in power with increasing take-off angle 327 islarger at higher frequencies. For example, the high frequency power ofthe vertical far-field signature 400(1) is larger than the highfrequency power of the other angle-dependent far-field signatures400(2-4), which correspond to larger take-off angles 327. The signalprocessing unit 132 may therefore form de-signatured traces using thetraces and the angle-dependent far-field signatures 400(1-4), in amanner described in detail below. Consequently, the high frequencycontent in the de-signatured traces may be increased, and amplitudeand/or phase errors in the de-signatured traces may be reduced oreliminated.

However, in various alternative embodiments, the power in theangle-dependent far-field signatures 400(1-4) may not vary uniformlywith take-off angle 327 and/or frequency. In one illustrativeembodiment, the sources 300(1-2) may be deployed deliberately oraccidentally such that the maximum power output is not in the verticaldirection and such that the power can decrease as well as increase withsome take-off angles 327. For example, the sources 300(1-2) may bedeployed in tilted and/or tuned arrays. In another illustrativeembodiment, a ghost may be included in the far-field signatures400(1-4), in which case the high frequency notch may increase withtake-off and emergent angles 327, 328, as will be appreciated by thoseof ordinary skill in the art. As discussed above, amplitude and/or phaseerrors caused, at least in part, by the variation of the far-fieldsignatures 400(1-4) may be reduced or eliminated by formingde-signatured traces using the traces and the angle-dependent far-fieldsignatures 400(1-4).

Referring now to FIG. 5A, a diagram of one embodiment of anangle-dependent de-signaturing process is shown. The plurality ofangle-dependent far-field signatures 400(1-4) are formed (at 500), inthe manner described above. For example, in one embodiment, theplurality of angle dependent de-signature filters are formed (at 500) tocorrect each of the plurality of angle dependent far-field signatures toa target wavelet. The traces are associated (at 510) with one or more ofthe plurality of angle-dependent far-field signatures 400(1-4). In oneembodiment, associating (at 510) the plurality of angle-dependentfar-field signatures 400(1-4) with the traces includes associating (at510) the traces with one or more take-off angles 327. For example, thetraces may be associated (at 510) with one or more take-off angles 327by forming a common receiver station gather, in a manner known to thoseof ordinary skill in the art. The traces are de-signatured (at 520)using the plurality of angle-dependent far-field signatures 400(1-4) andthe associated traces. In one embodiment, de-signaturing (at 520) thetraces includes forming a plurality of angle-dependent de-signaturefilters using the plurality of angle-dependent far-field signatures400(1-4) and applying the plurality of angle-dependent de-signaturefilters to the traces.

Referring now to FIG. 5B, one embodiment of a first exemplary processfor directional de-signature is shown. In the embodiment illustrated inFIG. 5B, directional de-signature includes applying (at 540) a verticalde-signature filter to the traces. Applying (at 540) the verticalde-signature filter may include selecting a vertical signature, e.g. theangle-dependent signature 400(1) shown in FIG. 4, and forming a verticalde-signaturing filter from the selected vertical signature. For example,a common receiver station gather typically includes traces from manyshots and so it may not be possible to apply the directionalde-signature to the trace corresponding to each shot individually. Inthis case, applying (at 540) the vertical de-signature filter to eachtrace may, at least in part, account for shot-to-shot variations. Oncethe shot-to-shot variations have been, at least in part, removed byapplying (at 540) the vertical de-signature, applying (at 520) thedirectional de-signature may include applying an average directivitycorrection that does not vary substantially from shot to shot.

As described in more detail below, one or more ghost signals may beremoved (at 545) from the traces. In one embodiment, common receiverstation gathers may be formed (at 548). By forming (at 548) the commonreceiver station gathers, the dips on the data represent the take-offangle 327 at the source 300(1-2), as will be appreciated by those ofordinary skill in the art having benefit of the present disclosure. Aportion of the traces may then be interpolated (at 550). For example,the portion of the traces may be interpolated (at 550) to avoidaliasing, as will be appreciated by those of ordinary skill in the art.The traces may then be transformed (at 555) from a XT domain to a τ-Pdomain. Transforming (at 555) the traces from the XT domain to the τ-Pdomain allows each trace to be associated (at 558) with a respectivetake-off angle 327 and an appropriate far-field signature.

The plurality of angle-dependent de-signature filters may be applied (at560) to the transformed traces and the resulting de-signatured tracesmay be transformed (at 570) from the τ-P domain to the XT domain. In oneembodiment, transforming (at 555) the traces from the τ-P domain to theXT domain may include removing (at 580) the interpolated portion of thetraces.

A second exemplary process for directional de-signature, shown in FIG.5C, includes forming (at 582) a two-dimensional de-signaturing filterrepresentative of the plurality of angle-dependent de-signature filtersusing the plurality of angle-dependent far-field signatures 400(1-4) andapplying (at 584) the two-dimensional de-signaturing filter to thetraces. In one embodiment, the two-dimensional de-signaturing filter isformed (at 582) by generating (at 586) a τ-P transform domain consistingof a spike on each of a plurality of traces. Each trace represents adifferent take-off angle 327. In one alternative embodiment, the τ-Ptransform domain may consist of an appropriately band-limited wavelet onthe traces. For example, in one embodiment, the spike and/or waveletwill typically be at the same τ time, e.g. at 2000 ms in a 4000 ms τ-Ptransform domain.

Appropriate de-signature operators may be formed (at 588) and convolved(at 590) with the traces, which may include the spike and/or thewavelet, for the corresponding take-off angle 327. For example, thede-signature operator corresponding to a take-off angle 327 of about 30degrees may be convolved (at 590) with the traces corresponding to atake-off angle 327 of about 30 degrees. The two-dimensionalde-signaturing filter may then be formed (at 582) by, for example,inverse transforming (at 592) the τ-P transform domain to the XT domain.However, as will be appreciated by those of ordinary skill in the art,the present invention is not limited to the aforementioned process offorming (at 582) the two-dimensional de-signaturing filter. Forming (at582) the two dimensional filter generally involves capturing the impulseresponse in XT space of the application of the angle dependentde-signature filters in τ-P space and any desirable process ofaccomplishing this may be used.

In one embodiment, applying (at 584) the two-dimensional de-signaturingfilter to the traces may include convolving the two-dimensionalde-signaturing filter with the traces sorted in the common receiverstation gathers. In one embodiment, convolving the two-dimensionalde-signaturing filter with the traces may be done in the XT domain.However, it will be appreciated by those of ordinary skill in the artthat the present invention is not limited to applications in the XTdomain. For example, in alternative embodiments, the two-dimensionalde-signaturing filter may be applied to the traces in the f-k domainand/or convolved with the traces in the f-x domain.

Referring now to FIG. 6, various interference signals that may form aportion of the near-field signature are shown. In one embodiment, the atleast one notional signature may be generated from a near-fieldsignature received by the near-field receivers 325(1-2) by removing atleast a portion of interference from other signals. For one example, theseismic sources 300(1-2) may provide interference signals 600(1-2),which may interfere with the direct arrival acoustic signal 320(1-2)shown in FIG. 3. For example, the seismic source 300(1) provides theinterference signal 600(1) to the near-field receiver 325(2). Foranother example, the seismic source 300(2) provides the interferencesignal 600(2) to the near-field receiver 325(1). For yet anotherexample, the seismic sources 300(1-2) may provide ghost signals 610(1-2)that are reflected from a sea surface 620. The ghost signals 610(1-2)may be received by the near-field receivers 325(1-2) and may interferewith the direct arrival acoustic signal 320(2) shown in FIG. 3. Althoughnot shown in FIG. 6, those of ordinary skill in the art will appreciatethat the signals in FIG. 6 may have corresponding take-off and emergentangles.

To reduce or eliminate the interference caused by the aforementionedsignals, the signal processing unit 132 may generate the aforementionedat least one notional signature by processing the near-field signaturereceived by the near field receivers 325(1-2) to remove the interferencefrom the interference signals 600(1-2), the ghost signals 610(1-2), andany other like signals. In one embodiment, the signal processing unit132 may record the near-field signature and/or the at least one notionalsignature on tape for subsequent processing.

In one embodiment, the signal processing unit 132 may also remove thereceiver ghost signals 630(1-2), as shown in FIG. 7. The receiver ghostmay vary as the emergent angle (328) varies. A common source gather,also known as a common shot gather, may be formed (at 710) from thetraces. By forming (at 710) the shot gather, the dips in the datarepresent the emergent angle 328 at the receiver 310. The traces aretransformed (at 715) from a XT domain to a τ-P domain so that each tracecan be associated (at 717) with a single emergent angle. In oneembodiment, transforming (at 715) the traces from the XT domain to theτ-P domain may include interpolating a portion of the traces. The tracesare then associated (at 717) with the emergent angle.

At least one appropriate receiver ghost filter is applied (at 720) tothe transformed traces and the resulting filtered traces may betransformed (at 730) from the τ-P domain to the XT domain. In oneembodiment, transforming (at 730) the traces from the τ-P domain to theXT domain may include removing the interpolated portion of the traces.

In an alternative embodiment, removing the receiver ghost signals630(1-2) from the traces includes forming a two-dimensional ghostremoval filter representative of the plurality of receiver ghost signals630(1-2) and applying the two-dimensional ghost removal filter to acommon source gather. In alternative embodiments, the two-dimensionalghost removal filter may also be applied in other domains such as thef-x or f-k domains, as will be appreciated by those of ordinary skill inthe art.

This concludes the detailed description. The particular embodimentsdisclosed above are illustrative only, as the invention may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein.Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A method, comprising: determining a plurality of far-field signaturesrepresentative of a plurality of seismic signals having a plurality oftake-off angles; associating a plurality of traces representative of aplurality of reflections of the seismic signals with the plurality offar-field signatures; and forming a plurality of de-signatured tracesfrom the plurality of traces and the plurality of associated far-fieldsignatures.
 2. The method of claim 1, wherein forming the plurality offar-field signatures comprises determining at least one notionalsignature using at least one near-field detector.
 3. The method of claim2, wherein forming the plurality of far-field signatures comprisesforming the plurality of far-field signatures from the at least onenotional signature, the plurality of take-off angles, and a model. 4.The method of claim 1, wherein associating the plurality of traces withthe plurality of far-field signatures comprises associating theplurality of traces with the plurality of take-off angles.
 5. The methodof claim 4, wherein associating the plurality of traces with theplurality of take-off angles comprises gathering a portion of the tracescorresponding to a common receiver station.
 6. The method of claim 1,wherein forming the plurality of de-signatured traces comprises forminga plurality of angle-dependent de-signature filters using the far-fieldsignatures.
 7. The method of claim 6, wherein forming the plurality ofde-signatured traces comprises applying the plurality of angle-dependentde-signature filters to the plurality of traces.
 8. The method of claim7, wherein applying the plurality of angle-dependent de-signaturefilters to the plurality of traces comprises transforming the pluralityof traces from an XT domain to a τ-P domain.
 9. The method of claim 8,wherein transforming the plurality of traces from the XT domain to theτ-P domain comprises interpolating a portion of the traces beforetransforming the traces to the τ-P domain.
 10. The method of claim 8,wherein applying the plurality of angle-dependent de-signature filtersto the plurality of traces comprises applying the plurality ofangle-dependent de-signature filters to the plurality of transformedtraces in the τ-P domain to form the de-signatured traces.
 11. Themethod of claim 10, wherein applying the plurality of angle-dependentde-signature filters to the plurality of traces comprises transformingthe de-signatured traces from the τ-P domain to the XT domain.
 12. Themethod of claim 11, wherein transforming the de-signatured traces fromthe τ-P domain to the XT domain comprises removing the interpolatedportion of the de-signatured traces after transforming the de-signaturedtraces from the τ-P domain to the XT domain.
 13. The method of claim 6,wherein forming the plurality of de-signatured traces comprises applyinga vertical de-signature filter selected from the plurality of far-fieldsignatures to the traces.
 14. The method of claim 13, further comprisingapplying an average directivity correction to the plurality of traces.15. The method of claim 1, further comprising removing at least onereceiver ghost from the plurality of traces.
 16. The method of claim 15,wherein removing the at least one receiver ghost comprises: associatingthe plurality of traces with a plurality of emergent angles;transforming the traces from an XT domain to a τ-P domain; applying atleast one receiver ghost filter to the transformed traces; andtransforming the traces from the τ-P domain to the XT domain.
 17. Themethod of claim 1, wherein forming the plurality of de-signatured tracescomprises forming the plurality of de-signatured traces using atwo-dimensional filter.
 18. The method of claim 17, wherein forming theplurality of de-signatured traces using the two-dimensional filtercomprises: forming the two-dimensional filter; and applying thetwo-dimensional filter to the traces.
 19. The method of claim 18,wherein forming the two-dimensional filter comprises: generating a τ-Ptransform domain; forming at least one de-signature operatorcorresponding to at least one take-off angle; convolving the at leastone de-signature operator with at least one trace corresponding to theat least one take-off angle; and inverse transforming the τ-P transformdomain to an XT domain.
 20. The method of claim 17, wherein forming theplurality of de-signatured traces comprises forming the plurality ofde-signatured traces using a two-dimensional ghost removal filter.
 21. Amethod, comprising: generating a plurality of seismic signals having aplurality of take-off angles using at least one seismic source;receiving a plurality of reflections of the seismic signals at aplurality of receivers, wherein the plurality of reflected seismicsignals have a corresponding plurality of emergent angles at theplurality of receivers; recording a plurality of traces representativeof the reflected seismic signals; and forming a plurality ofde-signatured traces from the plurality of seismic signals, theplurality of traces, the plurality of take-off angles, and the pluralityof emergent angles.
 22. The method of claim 21, wherein forming theplurality of de-signatured traces comprises associating each of theplurality of traces with at least one of the plurality of takeoffangles.
 23. The method of claim 22, wherein associating each of theplurality of traces with at least one of the plurality of takeoff anglescomprises gathering a portion of the traces corresponding to a commonreceiver.
 24. The method of claim 22, wherein forming the plurality ofde-signatured traces comprises forming at least one notional signatureusing the seismic signal.
 25. The method of claim 24, wherein formingthe at least one notional signature comprises receiving the seismicsignal at a near-field receiver proximate the seismic source.
 26. Themethod of claim 25, wherein receiving the seismic signal at a near-fieldreceiver proximate the seismic source comprises receiving the seismicsignal at a near-field receiver about 1 meter from the seismic source.27. The method of claim 24, wherein forming the at least one notionalsignature comprises removing seismic signals generated by other seismicsources from the seismic signal received by the near-field receiver. 28.The method of claim 24, wherein forming the at least one notionalsignature comprises removing ghost signals from the seismic signalreceived by the near-field receiver.
 29. The method of claim 24, whereinforming the plurality of de-signatured traces comprises forming aplurality of far-field signatures using the at least one notionalsignature.
 30. The method of claim 29, wherein forming the plurality offar-field signatures using the at least one notional signature comprisescalculating the plurality of far-field signatures for each of theplurality of takeoff angles.
 31. The method of claim 30, wherein formingthe plurality of de-signatured traces comprises forming a plurality ofangle-dependent filters from the plurality of far-field signatures. 32.The method of claim 31, wherein forming the plurality of de-signaturedtraces comprises applying the plurality of angle-dependent filters tothe plurality of traces.
 33. The method of claim 32, wherein applyingthe plurality of angle-dependent filters to the plurality of tracescomprises applying the plurality of angle-dependent filters to theplurality of traces in a τ-P domain.
 34. The method of claim 21, whereinforming the plurality of de-signatured traces comprises forming theplurality of de-signatured traces comprises using a two-dimensionalfilter.
 35. An apparatus, comprising: a storage unit for storing datarepresentative of a seismic signal, a plurality of traces representativeof a plurality of reflected seismic signals, and a plurality of take-offangles and at least one notional signature corresponding to the seismicsignal; and a processor capable of forming a plurality of de-signaturedtraces using the seismic signal, the plurality of traces, the take-offangles, and the at least one notional signature.
 36. The apparatus ofclaim 35, wherein the processor is capable of forming a plurality offar-field signatures using the at least one notional signature and theplurality of take-off angles.
 37. The apparatus of claim 36, wherein theprocessor is capable of associating the traces with the plurality oftake-off angles.
 38. The apparatus of claim 37, wherein the processor iscapable of forming at least one common receiver station gather using theplurality of traces.
 39. The apparatus of claim 36, wherein theprocessor is capable of forming a plurality of angle-dependent filtersformed from the plurality of far-field signatures.
 40. The apparatus ofclaim 39, wherein the processor is capable of applying the plurality ofangle-dependent filters to form the plurality of de-signatured traces.41. The apparatus of claim 40, wherein the processor is capable ofapplying the plurality of angle-dependent filters to form the pluralityof de-signatured traces in a τ-P domain.
 42. The apparatus of claim 41,wherein the processor is capable of interpolating a portion of thetraces before applying the plurality of angle-dependent filters to formthe plurality of de-signatured traces in the τ-P domain.
 43. Theapparatus of claim 42, wherein the processor is capable of removing theinterpolated portion of the traces after applying the plurality ofangle-dependent filters to form the plurality of de-signatured traces inthe τ-P domain.
 44. The apparatus of claim 39, wherein the processor iscapable of selecting one of the plurality of angle-dependent filtersrepresentative of a vertical far-field signature selected from theplurality of far-field signatures.
 45. The apparatus of claim 44,wherein the processor is capable of performing a vertical de-signatureprocess using the vertical far-field signature.
 46. The apparatus ofclaim 35, wherein the processor is capable of forming a two-dimensionalfilter.
 47. An article comprising one or more machine-readable storagemedia containing instructions that when executed enable a processor to:determine a plurality of far-field signatures representative of aplurality of seismic signals having a plurality of take-off angles;associate a plurality of traces representative of a plurality ofreflections of the seismic signals with the plurality of far-fieldsignatures; and form a plurality of de-signatured traces from theplurality of traces and the plurality of associated far-fieldsignatures.
 48. The article of claim 47, wherein the one or moremachine-readable storage media contain instructions that when executedenable the processor to determine at least one notional signature usingat least one near-field detector.
 49. The article of claim 47, whereinthe one or more machine-readable storage media contain instructions thatwhen executed enable the processor to associate the plurality of traceswith the plurality of take-off angles.
 50. The article of claim 47,wherein the one or more machine-readable storage media containinstructions that when executed enable the processor to form a pluralityof angle-dependent de-signature filters using the far-field signatures.